buffer: align chunks on 8-byte boundary

When slicing global pool - ensure that the underlying buffer's data ptr
is 8-byte alignment to do not ruin expectations of 3rd party C++ addons.

NOTE: 0.10 node.js always returned aligned pointers and v0.12 should do
this too for compatibility.

PR-URL: nodejs/node-v0.x-archive#9375
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

Author: indutny

lib/buffer.js

@@ -86,6 +86,12 @@ function Buffer(subject, encoding) {
                             poolOffset,
                             poolOffset + this.length);
     poolOffset += this.length;
+
+    // Ensure aligned slices
+    if (poolOffset & 0x7) {
+      poolOffset |= 0x7;
+      poolOffset++;
+    }
   } else {
     alloc(this, this.length);
   }